Torsional severe plastic deformation method for metal bar, employing surface polishing to improve mechanical properties of metal bar

ABSTRACT

The present invention relates to a torsional severe plastic deformation method for a metal bar to which surface polishing is applied to the metal bar to improve the mechanical properties of the metal bar. According to an embodiment of the present invention, there is provided a torsional severe plastic deformation method for a metal bar, which includes: applying torsion to a metal bar; and removing a surface defect on the surface of the metal bar, the surface defect being caused by the applying of torsion, wherein the removing of the surface defect is carried out in a continuous manner in which the removing of the surface defect is performed together with the applying of torsion or in a discontinuous manner in which the applying of torsion is temporarily stopped and then the applying of torsion is performed, and the removing of a surface defect increases the amount of torsional rotation or the shear strain applied to the metal bar.

TECHNICAL FIELD

The present invention relates to a torsional severe plastic deformationmethod for a metal bar, to which surface polishing is applied, and moreparticularly, to a torsional severe plastic deformation method for ametal bar that is capable of improving the mechanical properties of amaterial by forming a gradient structure through shear deformationformed by applying a huge shear stress through torsion whilesubstantially maintaining the shape of the metal bar, and by ultrafinecrystallization or nano-crystallization of fine structures of a metalpipe material.

BACKGROUND ART

When plastic deformation is applied to a metal material, a dislocationcell structure having a small boundary angle is formed, and a phenomenonoccurs in which the greater the amount of plastic deformation, thegreater the crystal grain boundary angles of dislocation cellsub-grains, and the finer crystal grains gradually become. Using this,when a large plastic deformation is applied to a material and crystalgrains thereof are thereby ultrafinely crystallized andnano-crystallized, the material has remarkably improved mechanicalproperties (strength, hardness, polishing resistance, superplasticity,etc.). Thus, there is growing importance and demand for a processingmethod for producing ultrafine/nano crystal materials, beyond aconventional material processing method which is mainly for shapeformation.

Not only an amount of plastic deformation, such as compressive, tensile,and shear deformation, is important for the formation of ultrafine/nanocrystal grains, but also the design of a die is important such thatrepeated processes capable of applying a large amount of plasticdeformation may be performed and the shape of the material issubstantially the same before and after the process.

Severe plastic deformation methods such as an equal channel angularpressing (ECAP) process, a high-pressure torsion (HPT) process, anaccumulative roll bonding (ARB) process, and an equal channel angularrolling (ECAR) process, which meet the above requirements, have beendeveloped to date.

However, when an ultrafine/nano crystal material is formed, thephenomenon occurs in which the strength and hardness of the material areimproved, but ductility of the material decreases. A gradient structureof crystal grain sizes has been proposed as an alternative for solvingthe phenomenon of decreasing ductility. When a metallic material has agradient structure of the crystal grain sizes thereof, ductilityincreases in a region formed of large crystal grains, and strength andhardness are improved by a region formed of ultrafine/nano crystalgrains, and thus, opposed mechanical properties may both be achieved.Thus, for ultrafinely crystallized/nano-crystallized metal materials, agradient structure has emerged as a solution for the ductility decreaseproblem.

A gradient structure may be formed through a high-pressure torsion (HPT)process among the existing sever plastic processing methods, but thisprocessing method has a disadvantage that the size of the producedmaterial is limited because of requiring high pressure. Thus, a simpletorsion process is being required as a processing method for producing abulk material having a gradient structure.

Such a torsional extreme-plastic process is stopped by a shear fracturephenomenon, but there is a case of being stopped while sufficientplastic deformation is not applied, and thus, a method for delaying ashear fracture phenomenon is being required. A torsional processingmethod has a characteristic that the greater the distance from a centeraxis, the greater the amount of deformation applied. Thus, a defect iscaused on the surface to which the largest amount of deformation isapplied, and a shear fracture phenomenon proceeds at the point at whichthe defect occurs while a process proceeds.

DISCLOSURE OF THE INVENTION Technical Problem

An object of the present invention is to provide a torsional severeplastic deformation method for a metal bar with which larger deformationprocessing than existing torsional processing can be performed by addinga surface polishing process in a process of adding torsional severeplasticity to an existing metal bar, fine structures may be ultrafinelycrystallized or nano-crystallized, and the mechanical properties of themetal bar may be improved by forming a gradient structure of crystalgrain sizes.

Technical Solution

According to an embodiment of the present invention, there is provided atorsional severe plastic deformation method for a metal bar, whichincludes: applying torsion to a metal bar; and removing a surface defecton the surface of the metal bar, the surface defect being caused by theapplying of torsion, wherein the removing of the surface defect iscarried out in a continuous manner in which the removing of the surfacedefect is performed together with the applying of torsion or in adiscontinuous manner in which the applying of torsion is temporarilystopped and then the applying of torsion is performed, and the removingof a surface defect increases the amount of torsional rotation or theshear strain applied to the metal bar.

Advantageous Effects

According to a torsional severe plastic deformation method of thepresent invention, a gradient structure of crystal grain sizes may beformed by applying shear deformation while maintaining the shape of ametal bar, and fine structures may be ultrafinely crystallized, andthus, the mechanical properties of the material may be improved. Inaddition, the torsional severe plastic deformation method of the presentinvention may improve the degree of gradient and the degree of finecrystallization of fine structures by applying a more amount ofdeformation than existing torsional deformation through surfacepolishing. In addition, the torsional severe plastic deformation methodof the present invention may adjust torsional deformation and mechanicalproperties by adjusting a rotation speed.

In addition, the torsional severe plastic deformation method of thepresent invention is capable of freely adjusting the amount ofdeformation applied to a material by adjusting the rotation speed of adie, and thus is easy to reinforce the physical properties of metal barsand adjust fine structures of the metal bars.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a torsional severe plastic deformationmethod and processing equipment according to an embodiment of thepresent invention.

FIG. 2 is a view illustrating a cross-sectional surface of a specimenused in an embodiment of the present invention.

FIG. 3 is a view illustrating each of metal bars before and after atorsional severe plastic deformation according to an embodiment of thepresent invention.

FIG. 4 is a result of comparing the hardness of a metal bar according toan embodiment of the present invention with that of an existing metalbar.

FIG. 5 is a schematic view illustrating a region for which an analysishas been performed on a metal bar processed according to an embodimentof the present invention.

FIG. 6 is a result of electron back-scatter diffraction (EBSD) analysison a metal bar on which simple torsional processing is completed.

FIG. 7 is a result of electron back-scatter diffraction (EBSD) analysison a metal bar according to an embodiment of the present invention.

FIG. 8 is a table comparing simple torsion processing on a metal bar andan embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail onthe basis of preferred embodiments of the invention. However, thefollowing embodiments are merely examples for helping understanding ofthe invention, and the scope of the invention is not reduced or limitedby the embodiments.

In the present invention, it is proposed that a process of continuouslyremoving surface defects is added to an existing torsion process todelay a shear fracture phenomenon, and processes for forming ultrafinecrystal grains or nanocrystal grains are improved by applying asufficient amount of plastic deformation to a material.

When applying the surface treatment of the present invention, muchgreater deformation by further delaying surface fracture than existingtorsional severe plastic deformation methods, and the degree ofultrafine crystallization or nano crystallization may be reinforced tobe finer.

FIG. 1 is a view illustrating a torsional severe plastic deformationmethod and processing equipment according to an embodiment of thepresent invention.

FIG. 2 is a view illustrating a cross-sectional surface of a specimenused in an embodiment of the present invention.

Referring to FIGS. 1 and 2, a torsional severe plastic deformationmethod for metal bars according to an embodiment of the presentinvention includes: a process of applying torsion to a metal bar; and aprocess of removing surface defects caused on the surface of the metalbar by the process of applying torsion, wherein the process of removingthe surface defects is carried out in a continuous manner of beingperformed along with the process of applying torsion or in adiscontinuous manner of being performed while temporarily stopping theprocess of applying torsion, and a torsional rotation amount and a shearstrain which are applied to the metal bar may be increased.

Specifically, the process of applying torsion may include: installing ametal bar between a pair of dies; and performing torsion of the metalbar by rotating at least one of the pair of dies.

For example, dies conforming to the shape of the metal bar are attachedon both upper and lower sides of the metal bar, a surface polishingoperation is performed while applying torsion to the metal bar, and agradient structure of crystal grain sizes may be formed whileultrafinely crystallizing or nano-crystallizing the fine structures ofthe metal bar.

Here, in the performing of torsion of the metal bar, the gradientstructure of crystal grain sizes may be formed in the metal bar by usingshear deformation formed by shear stress.

The process of removing surface defects may include a polishing process.

Here, the polishing process may be performed by using a silicon carbide(SiC) polishing paper or by using a polishing material in which thegreater a torsional rotation amount or a shear strain, the smaller thesurface roughness.

In addition, in the surface polishing process, the metal bar isseparated from the dies, and then the surface of the metal bar may bepolished in a discontinuous manner or the surface of the metal bar fixedby the dies may be polished in a continuous manner.

At this point, in the surface polishing process, a silicon carbide (SiC)grinding paper having the size of ×400, ×600, ×800 and ×1200 may beused. The surface roughness of each silicon carbide grinding paper is 22μm, 15 μm, 10 μm or 5 μm, and when surface polishing is performed with aspecific silicon carbide polishing paper, the surface roughness of thespecimen (metal bar) is formed to be the corresponding roughness orbelow.

In addition, when discontinuously performing the surface polishingprocess, the surface roughness of the polished surface formed during thetorsion process may be reduced through polishing by at mostapproximately 5 μm per one-cycle surface polishing process.

If a procedure proceeds to a next process without polishing, in thesurface polishing process, the surface defects formed by torsion to havea roughness of at most approximately 5 μm, there occurs a problem inthat a shear fracture phenomenon proceeds from the remaining defects andthe torsion process is stopped. In addition, when the surface polishingis excessively performed even though the surface defects have beenremoved, there may occur a problem in that the cross-sectional area ofthe specimen decreases.

When proceeding the torsion process at a constant rotation speed, theshear fracture phenomenon occurs according to the inherent property of amaterial and the maximum torsional rotation amount or the maximum shearstrain at which deformation is stopped may be different.

In the present invention, in order to increase the maximum torsionalrotation amount or the maximum shear strain of each material by delayingthe shear fracture phenomenon through the surface polishing, the surfacepolishing operation may be performed in the order of silicon carbide(SiC) polishing papers of ×400, ×600, ×800 and ×1200 in the process ofapplying torsion. Here, as approaching the maximum torsional rotationamount or the maximum shear strain of a metal bar, surface defects atthe point to which stress (stress) is concentrated are removed byincreasing the frequency or the number of surface polishing operations,and thus, the efficiency of increasing the maximum torsional rotationamount or the maximum shear strain may be maximized.

The torsional severe plastic deformation method of the present inventionapplies more plastic deformation by increasing the maximum torsionalrotation amount or the maximum shear strain than existing metal bars,and achieves improved ultrafine crystallization or nano-crystallizationand the formation of a gradient structure of crystal grain size, wherebyit is easy to reinforce the property of the metal bar and to adjust thefine structures of the metal bar. In addition, the torsional severeplastic deformation method of the present invention may improve thestrength and hardness by applying more plastic shear deformation thanthe existing simple torsion process through surface polishing.

FIG. 3 is a view illustrating each of metal bars before and after aprocess of applying torsion according to an embodiment of the presentinvention.

FIG. 4 is a result of comparing the hardness of a metal bar according toan embodiment of the present invention with that of an existing metalbar. Referring to FIG. 4, it may be confirmed that a metal bar accordingto an embodiment of the present invention receives a more deformationamount by surface polishing and has an improved hardness.

FIG. 5 is a schematic view illustrating a region for which an analysishas been performed on a metal bar processed according to an embodimentof the present invention.

FIG. 6 is a result of electron back-scatter diffraction (EBSD) analysison a metal bar on which simple torsion processing is completed. Here, ananalysis has been performed on the basis of the position in FIG. 5.

FIG. 7 is a result of electron back-scatter diffraction (EBSD) analysison a metal bar according to an embodiment of the present invention.Here, an analysis has been performed on the basis of the position inFIG. 5.

FIG. 8 is a table showing results of comparison between simple torsionprocessing on a metal bar and an embodiment of the present invention.Referring to FIG. 8, the degree of ultrafine crystallization ornano-crystallization on a metal bar according to an embodiment of thepresent invention may be confirmed.

According to a torsional severe plastic deformation method of thepresent invention, a gradient structure of the crystal grain sizes canbe formed by applying shear deformation on a material while maintainingthe shape of the metal bar, and ultrafine crystallization of finestructures may be achieved, and thus, the mechanical properties of thematerial may be improved.

In addition, the torsional severe plastic deformation method of thepresent invention may improve the degree of gradient and the degree offine crystallization of fine structures by applying a more amount ofdeformation than existing deformation through surface polishing.

In addition, the torsional severe plastic deformation method of thepresent invention may adjust torsional deformation and mechanicalproperties by adjusting a rotation speed.

In addition, the torsional severe plastic deformation method of thepresent invention may freely adjust the amount of deformation applied toa material by adjusting the rotation speed of a die, thereby easilyreinforcing the physical properties of metal bars and adjusting finestructures.

So far, the technical idea of the present invention has been describedwith reference to the accompanying drawings, but this is for merelyillustrating a preferred embodiment of the present invention rather thanlimiting the present invention. In addition, it is obvious that any oneskilled in the art could carry out various modification and imitationwithout departing from the scope of the technical idea of the presentinvention.

1. A torsional severe plastic deformation method for a metal bar,comprising: applying torsion to a metal bar; and removing a surfacedefect on the surface of the metal bar, the surface defect being causedby the applying of torsion, wherein the removing of the surface defectis carried out in a continuous manner in which the removing of thesurface defect is performed together with the applying of torsion or ina discontinuous manner in which the applying of torsion is temporarilystopped and then the applying of torsion is performed, and the removingof a surface defect increases the amount of torsional rotation or theshear strain applied to the metal bar.
 2. The torsional severe plasticdeformation method for a metal bar of claim 1, wherein the removing of asurface defect comprises a polishing process.
 3. The torsional severeplastic deformation method for a metal bar of claim 2, wherein a siliconcarbide (SiC) grinding paper is used for the polishing process.
 4. Thetorsional severe plastic deformation method for metal bars of claim 2,wherein the polishing process is performed by using a polishing materialthat can reduce the surface roughness of the metal bar according to theincrease in the amount of torsional rotation or shear strain.
 5. Thetorsional severe plastic deformation method for a metal bar of claim 4,wherein when the polishing process is performed in a discontinuousmanner, the polishing process is performed so that the surface roughnessof the metal rod is 5 um or less per torsion process.